Interactions Between Carbon-based Materials And Terahertz Waves

Carbon element is one of the basic elements of life material and various kinds of organic matter.Carbon-based materials,such as graphite,graphene and carbon nanotubes have been well known,but the interactions between them and terahertz waves are seldom studied.In this dissertation,we mainly focus on two aspects: graphite terahertz generation and graphene terahertz modulation,which not only reveal the physical properties of carbon materials from a new angle of view,but also demonstrate the application potential of graphene in terahertz modulation.The research of graphite terahertz generation originally began in 2009,then a few papers followed up,but so far there is no unified conclusion on the physical mechanism.We electrically modulate terahertz radiation in situ,and find that the amplitudes of terahertz radiation vary continuously with the change of voltage,and the terahertz waveforms reverse when the sign of voltage changes,thus reveal the mechanism-surface field driving photon induced carriers into transient current to radiate terahertz wave.In addition,we find that,unlike semiconductors,the terahertz generation from graphite show a quicker saturation for surface-field tuning,and the saturation amplitudes under positive and negative bias are different,the ratio of which is close to 2.3.Based on the simulation results of first principles,we quantitatively prove that this stems from effective mass difference between electrons and holes within graphite.Graphene plasmon is a popular research direction in recent years.The interactions between graphene plasmon and terahertz waves are also a fresh new research topic,which may contain many unknown physical problems waiting for exploration.At present,researchers have found some unique physical properties of graphene plasmon that are different from those of traditional semiconductor two-dimensional electron gases,and have demonstrated its application potential in terahertz regime.Our research on this subject can be divided into two aspects:For one thing,we optimize the structures and dimensions of graphene metamaterials through computer simulation,so as to realize a large-depth and frequency-selective modulation in 0.1~3 THz range with graphene plasmon.Three complementary structures of graphene metamaterials are simulated using CST software: anti-dots,anti-ribbons and Complementary Split-Ring Resonantors(CSRR).Though comparison,we find that the modulation depth of CSRR structure is the largest,which provides a new optimum scheme for the design of graphene-based THz modulators and infrared sensors;For another,we fabricate a terahertz frequency-selective modulator with CSRRpatterned graphene metamaterials.Experimental results show that the modulator has large modulation depths for two polarizations,both of which are up to 35%,which is the highest record in literature so far for pure graphene structure.Through pattern design,we shift the resonance frequency of graphene plasmon to 0.5 THz.To the best of our knowledge,this is the lowest-frequency graphene plasmon resonance reported in literature.Our work further promotes the application of graphene plasmon in THz modulation,because 0.1~3THz is currently the working band of most THz-TDS systems.In addition,our work paves the way for the optoelectronic application of graphene,as 0.1THz is very close to the frequency range of electric application.